1. Field of the Invention
The present invention relates to a flat type resinoid grinding wheel, and more particularly to an improvement in the flat type resinoid grinding wheel which contains inorganic substance having a lower hardness value than the abrasive grains.
2. Prior Art
A flat type resinoid grinding wheel is, as has been well know, produced in such process that phenol resin is added to and mixed with hard abrasive grains, such as fused alumina, sintered alumina, silicon carbide, and zirconia, etc., the mixture poured into a mold, preheated, pressed, then burned to complete it. Heretofor, to improve the grinding performance of such grinding wheel, all that has been studied is mainly how to obtain higher hardness abrasive grains; as a result, zirconia abrasive grains have been recognized as a better material than fused alumina abrasive grains, and further a diamond-cutter formed by fusedly sticking diamond abrasive grains onto a metal plate has been utilized. However, in a flat type resinoid grinding wheel, even the hardness value of its abrasive grains is set fairly high; as long as it has low porosity, the grinding efficiency resulting has not been improved. Therefore, efforts to improve the grinding efficiency by using super hardness abrasive grains seems to have reached its limit.
The present inventor has found that when the mixture consisting of low hardness natural rock mineral and abrasive grains is pressure molded (This is, however, entirely opposed to the method of the prior art, in which higher hardness abrasive grains are sought to be employed.), extremely high performance grinding wheels could be obtained. Such grinding wheels, already applied for as a patent by the present inventor under U.S. Ser. No. 179,742, now abandoned showed over two time superior performance in plane grinding done by an offset grinding wheel, and also showed over six times superior performance done by cutting wheels to the prior art's one. Mainly described in the patent specification are matters related to offset type grining wheels and cutting wheels, while an application for the invention to a flat type resinoid grinding wheel is merely suggested. However, the flat type resinoid grinding wheel of the present invention may widely vary in size in contrast to an offset grinding wheel and cutting wheel; that is, such large size grinding wheels reaches to over 50 mm in thickness, 600 mm in diameter and 30 kg in weight. In such large size flat type grinding wheel, the abrasive grains are the largest part of the manufacturing cost; therefore, it is important to minimize the amount of abrasive grains to cut the costs.
Generally, in the case of thick and large-diameter flat type resinoid grinding wheels, it is extremely important to keep well balanced three principal elements--abrasive grains, bond, and pores--which are required in manufacturing a grinding wheel. More specifically, since abrasive grains act as a "cutter", they need to be very hard and sharp and also be equal in size to one another; since the bond supports the "cutter", it needs to have suitable adhesion to prevent pore blockage which might occur when the adhesion is too strong and also to prevent rapid wear which might occur when adhesion is too weak; and pores are required to take out the waste so as to prevent pore blockage, which might occur when pores are few, to reduce grinding performance. Consequently, it has been commonly believed that the harder abrasive grains are better material for a grinding wheel, and too much bond and too little number of pores are unfavorable because they cause pore blockage.